1. Field of the Invention
This invention relates to a method of steering a mining machine and is particularly concerned with the steering of a double-ended ranging drum mining machine.
2. Description of the Related Art
Such machines are used particularly in mining minerals, such as coal, where the desired mineral is extracted from a seam by a long-wall mining method. In the long-wall mining method the machine successively traverses a face which may be of the order of 250 m in length, cutting mineral as it goes. The machine carries a rotating cutting drum at each end of its ranging arms and one of the drums cuts, as a leading drum, the top of the seam while the other, the trailing drum, cuts the lower part of the seam.
It is necessary in order to maximise the economics of the mining operation to ensure that the fullest extraction of the desired mineral from the seam is taken, without there being any excursion by the cutting drum into the overlying or underlying strata. This is usually achieved by determining that a desired thickness of the mineral is left at the roof and the floor. Roof coal also helps in stabilizing the roof conditions. A typical roof thickness is of the order of 100 mm.
One way in which this roof thickness is maintained is by measuring the amount of natural gamma radiation emitted by the adjacent strata as described in our British Patent No 1 526 028. This radiation can be picked up by a gamma detector situated on the machine and the strength of the signal received is dependent on the attenuation of the signal by the quantity of roof left after the cutting operation. If the signal is attenuated too far as the thickness increases, then a correction steering signal can be given to alter the angle of the ranging arm to alter the cut so that a lesser thickness of roof is left.
However, in order to achieve this, it is also necessary to measure physically the roof step, i.e. the difference in roof heights between the previous pass cut roof, at which a roof coal thickness measurement is available, and the leading drum, in order that further steering of the drum can take place. Currently, this measurement is usually effected by using a roof follower attached to the ranging arm itself. This follower contacts the roof, cut on the previous pass, but adjacent to the drum, and physically follows its contours. As deviations occur, a transducer produces electrical signals which can be fed to a comparator for altering the angle of the ranging arm as necessary.
The steering may also be controlled by a factor which takes account of the inclination or tilt of the machine towards the face.
Particularly in deep seams, the roof follower, which is located in a vulnerable position close to the drum, may be a very long cantilevered arm and thus quite flexible and liable to damage, for example by mineral falling from the roof or by irregularities in the roof itself.
In our co-pending UK Patent Application No. 90 02956.2 (Publication No. 2241006A) we have described a method of measuring various parameters for steering respectively the leading and the trailing drums of a mineral mining machine using a current cut follower at or near the centre of the machine. However, although both algorithms were shown not to be subject to propagating errors and the trailing drum algorithm was shown to be subject to geometrical transducer errors alone, the leading drum algorithm was also dependent on multiple coal thickness measurements and their associated errors.
It is an object of the present invention to provide an alternative method of steering which employs an additional roof follower placed in the previous cut in order to reduce the errors of the leading drum algorithm. Through the introduction of a strategically placed previous cut roof follower, or roof height sensor, at the same along face position as a current cut roof follower an algorithm will be developed which will reduce all steering control variables to local height differences (effectively local cut roof height differences), the angles and coal thickness measurements.